The invention is directed to an optical positioning system for at least two picture elements wherein at least two light sources emit light beams for generating the picture elements in an image plane.
Picture element positioning systems are known for a great variety of applications and in many embodiments. A significant group thereof serves for line-by-line scanning of surfaces, either for acquiring or for writing information. The dominating feature is thereby a continuously rotating, mechanical light deflector element.
In order to enhance the precision of the mechanical deflection motion, the combination of a specific category of mechanical deflection systems with an additional deflection possibility via acousto-optical deflectors is known (German Published Application 24 43 379, U.S. Pat. No. 4,279,472). Scan systems such as oscillating mirrors or polygonal wheels wherein the deflection surface has only a small angle relative to the ray beam incident onto the deflector element are included in this category of mechanical deflection systems: only for such systems, namely, is the direction of a misdirection detected in the image plane or at an equivalent location transmitted onto the acousto-optical deflector lying in the pre-scan system independently of the deflection position.
This, however, is not the case for scan systems having, for example, mirror faces at 45.degree. relative to the rotational axis or prism deflectors, both charged parallel to the rotational axis.
Further, systems are known that select light bundles that differ in wavelength and can use them simultaneously or sequentially for scanning, for example JP-A-222817, wherein an optical positioning system employs a wavelength-dispersive light-deflecting element that is impinged upon in common by light beams of light sources wherein light beams are deflected in a first deflection direction within a deflection range according to the wavelength for positioning the picture elements.
In a laser printer that also belongs to the prior art, the ray beam emanating from a tunable semiconductor laser is guided with a scan motion along a straight scan line across a light-sensitive recording material on the recording drum of the laser printer, being guided with a hologram module (European Published Application 0 277 883). A f.theta. lens is also inserted into the beam path between the hologram module and the scan plane in order to focus the ray beam in the scan plane. The hologram member is a transparent polygon on which a desired plurality of hologram gratings are arranged, these being charged by the laser ray beam. In a simplest case, a hologram lens can also be provided instead of the hologram module. The hologram module and the hologram lens represent light-deflecting, wavelength-dispersive elements, i.e. elements whose diffraction is critically dependent on the light wavelength. A non-mechanical deflection of the ray beam is thus achieved that can occur with extremely high speeds. Versions of this positioning system are also known wherein the hologram module is simultaneously charged with a plurality of ray beams that, for example, emanate from three controllable lasers. The angles of incidence of the three ray beams on the hologram module can thereby be either identical or differ from one another.
What is disadvantageous in all of these known embodiments comprising a tunable laser and a holographic module or, respectively, a deflection element is that only a rather limited number of picture element positions can be set with high precision, this being inadequate, particularly for producing typographically demanding type matter from picture elements (pixels). The obtainable plurality of picture element positions in an optical positioning system of the applicable type is fundamentally defined by the spectral tuning range of the light source and the required spectral breadth for an exactly reproducible wavelength. For example, given a spectral tuning range of laser diodes that amounts to approximately 10 nm and given a typical temperature-dependency of the wavelength of 0.1-0.3 nm/K, a plurality of at most 1,000 exact image positions can be achieved even given a temperature stabilization to 0.1K, whereas many tens of thousands of positions must be typically resolved given scanners for the graphic industry.
The picture element positioning belonging to the prior art employing the assistance of acousto-optical deflectors without mechanical motion and only by varying an audio frequency that is generated in the acousto-optical deflector has the limitation that the deflector can only resolve a comparatively low number
of image positions and can only realize small deflection angles.
Known opto-mechanical positioning systems wherein the light-deflecting element rotates, in particular, around a rotational axis can have large usable deflection angles given high positioning precision. In particular, polygonal mirrors having faces that are arranged at 45.degree. relative to the rotational axis are advantageous since the 1:1 conversion of rotational angle into scan angle that can therewith be achieved allows a good positioning precision. Moreover, deflecting prisms having 90.degree. deflection (as the aforementioned polygonal mirrors) are insensitive to disruptions of the ideal mirror position, for example due to bearing wobble or vibration. A rotating pentaprism having a following scan objective is remarkably insensitive to tilt.
There is a desire to increase the usable scan speed in many areas, particularly in the graphic industry, in the case of the described, opto-mechanical deflectors that move at least one ray beam over the image plane or surface to be scanned in the scan mode. This increase can be generally achieved in that scanning is undertaken not only with one picture element but with a plurality of picture elements that are preferably guided at a constant spacing relative to one another. In the preferred opto-mechanical scan systems that are constructed with a polygonal mirror having faces at 45.degree. relative to the rotational axis or are constructed with at least one prism and that are charged with a ray beam lying in the rotational axis in the ideal case, it is not possible to expand a plurality of ray beams parallel to one another and, thus, necessarily lying partly outside the rotational axis, since a rotation of the deflecting element or deflector is expressed in rotation of the picture elements insofar as these do not lie in the optical axis or the rotational axis. If a plurality of picture elements in a region of the scan line were arranged on a line at a right angle vis-a-vis the scan direction, then this line would incline toward the scan line in accordance with the respective rotational angle.